Prior U.S. Pat. No. 4,342,734, issued Aug. 3, 1982 to Kumar and Das, discloses the preparation of thick, dense wafers of crystalline gamma-tetragonal boron by plasma-jet applying beta-rhombohedral boron in powder form onto a substrate and rapidly cooling the molten particles. The beta-boron powder had a particle size of -100 mesh. The patent makes no mention of the optical properties of the cooled deposit nor reference to the formation of optically black surfaces. The patent is also silent on other properties such as resistance to high temperature degradation or corrosion resistance to chemical or oxidizing environments.
Prior U.S. Pat. No. 3,231,416, issued Jan. 25, 1966, to Fuller, discloses the preparation of zirconia-boron ablation coatings by plasma-jet spraying. An example of a powder used in the '416 patent contained 60 volume percent zirconia (325 mesh) and 40 volume percent boron (100 mesh). The coatings were said to be capable of withstanding temperatures of about 2280.degree. C. during a ten second test period. This, in part, was attributed to good thermal emittance or ability to radiate heat away from the coated body. As in the '734 patent, no reference is made to the infrared or visible wavelength absorption or other properties which characterize the present invention.
Also of interest are prior U.S. Pat. Nos. 4,503,085, dated Mar. 5, 1985, issued to Dickson et al.; 4,526,618, issued July 2, 1985, to Keshavan et al.; and 4,696,855, dated Sept. 29, 1987, issued to Pettit, Jr., et al.